This invention relates generally to sputtering apparatus and methods and more particularly to sputtering apparatus and methods used in depositing material on a surface of an object.
As is known in the art, it is frequently desired to use sputtering apparatus to deposit selected materials, such as dielectric materials or conductive metals, over the surface of an object, such as a semiconductor wafer, at some stage in the processing of such object. Typically, the sputtering apparatus includes a chamber filled with a low pressure noble gas, such as argon. The material to be deposited, generally referred to as the "target," is disposed on, or adjacent to, an upper cathode electrode within the chamber, while the object is disposed on a lower pallet which serves as the anode electrode and which is also disposed within the chamber. When a suitable electrical potential is produced between the anode and cathode electrodes the argon ionize with the result that positively charged argon ions are attracted to the cathode electrode and hence such ions bombard the target. This bombardment of the target causes material to be dislodged therefrom and become deposited on the surface of the object.
In many applications it is desirable to clean the target prior to the deposition process. For example, in some applications where the object is a semiconductor wafer and where the sputtering apparatus is used to deposit a layer of a conductive material over the surface of such wafer it is sometimes desirable to clean the surface of the wafer from oxides or other contaminants prior to the deposition process. To clean the surface of the wafer the pallet holding the wafer is electrically coupled to the power supply as a cathode electrode with the target holder electrically disconnected from the power supply. The chamber's conductive walls serve as the anode electrode with the result that the argon ions are attracted to the cathode electrode, now the wafer holder, and thereby bombard the surface of the wafer, causing the contaminants thereon to be knocked out by the impinging argon ions and become deposited on the various surfaces within the chamber. In such wafer cleaning process, however, some of the contaminants become deposited on the surface of the target. In some sputtering apparatus a rotatable shutter is provided. The shutter is generally a planar-shaped apertured disk disposed within the chamber between the lower pallet and the upper target. The shutter is mechanically coupled to a servomechanism which positions the aperture therein between the target and the wafer during the deposition phase (i.e. positioning the shutter in the "open" position), but which rotates the shutter so that the aperture is moved to a different position and a nonapertured portion of the shutter is disposed between the target and the wafer (i.e. the closed position) to provide some shielding to the target against contaminants which are dislodged from the surface of the wafer during the wafer cleaning process. It is noted, however, that despite the inclusion of the shutter some target contamination may still occur with the result that cleaning of the target surface itself may be required prior to the deposition process.
In order to clean the target prior to the deposition process the power supply is switched so that the target becomes, in effect, the cathode, and the wafer pallet support the anode. The shutter remains in the "closed" position with the result that argon ions bombard the surface of the target and dislodge atoms on the contaminated surface thereof. Such dislodged atoms become deposited on the upper surface of the shutter, predominantly on the nonapertured portion of the shutter's upper surface disposed beneath the target. Having cleaned the target, the shutter is rotated to the "open" position so that the aperture is disposed between the target and the wafer. The power supply is again fed to the target holder and pallet so that the target is, in effect, the cathode, and the pallet, the anode. Therefore, argon ions bombard the target and dislodge material therefrom. Such dislodged material passes through the aperture and becomes deposited on the surface of the wafer.
While such apparatus and process have worked effectively with some target materials for other materials, such as refractory metals of titanium, tungsten and mixtures thereof, during the target cleaning process these other materials which adhere relatively well to the shutter surfaces when such surfaces are relatively clean do not adhere well to layers of material which build up on such surfaces after many target cleanings with the result that portions of such material, called "particulates," may break off from the upper surface of the shutter and fall onto the surface of the wafer during the deposition process. In order to avoid this the shutter must be cleaned periodically. Unfortunately, this cleaning requires that the apparatus be shut down, thereby causing a reduction in productivity and an increase in product cost.
As is also known in the art, some sputtering apparatus include more than one target. In such apparatus the aperture in the shutter is positioned under a selected target during the deposition process (i.e. an "open" position). The wafers are mounted on a pallet which is adapted to rotate about an axis, the targets being disposed in a circle through which the center of such axis passes. The wafers are arranged on the pallet in concentric circles, the axis passing through the center of the circles. In such apparatus atoms ejected from the selected target pass through the aperture in the shutter onto the wafers exposed by the shutter with the result that all wafers become deposited with target material as the pallet rotates through complete revolutions. In order to provide uniform deposition on the wafers the aperture must be shaped to compensate for the fact that wafers closer to the axis of rotation travel with a different velocity than wafers disposed near the outer periphery of the pallet. In some apparatus a uniformity shield (i.e., a mask having an aperture shaped to provide the desired compensation) is mounted between the target and the shutter. More particularly, the uniformity shield is mounted in fixed relationship below the target. Thus, each target has a fixed uniformity shield and such target is positioned above its associated uniformity shield. In such apparatus the uniformity shields are disposed below the so-called "dark space" region which is formed adjacent to the cathode target and hence are disposed in the plasma or glow region of the chamber with the result that target atoms dislodged therefrom during the deposition process become deposited on upper surfaces of the shields. Here again, for some materials, such as titanium, tungsten and mixtures thereof, adhesion to the upper surfaces of the shields is initially relatively good. However, as layers of such material build up on such surfaces of the shields the adhesion forces are reduced with the result that pieces of the material deposited on the shields' surfaces break off and fall onto the surface of the lower wafers. Thus, with such multiple target apparatus, in addition to the depositions which occur on the shutter during the target cleaning process, depositions occur on the surfaces of the uniformity shields during the actual wafer deposition process. It follows, then, that these shields are an additional source of contamination in the processing, thereby requiring still more frequent cleaning of the apparatus and resulting in loss of use of the apparatus, reduced productivity and increased product cost.